Derivative Instruments (Details) (USD $)	6 Months Ended			6 Months Ended	3 Months Ended		6 Months Ended													6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013	Jun. 30, 2014 Forward Starting Interest Rate Swap [Member] contract	Jun. 30, 2014 Interest Rate Swap [Member] Interest Expense [Member]	Jun. 30, 2013 Interest Rate Swap [Member] Interest Expense [Member]	Jun. 30, 2014 Interest Rate Swap [Member] Interest Expense [Member]	Jun. 30, 2013 Interest Rate Swap [Member] Interest Expense [Member]	Jun. 30, 2014 Interest Rate Swap 1 [Member]	Jun. 30, 2014 Interest Rate Swap 2 [Member]	Jun. 30, 2014 Interest Rate Swap 3 [Member]	Jun. 30, 2014 Interest Rate Swap 4 [Member]	Jun. 30, 2014 Interest Rate Swap 5 [Member]	Jun. 30, 2014 Interest Rate Swap 6 [Member]	Jun. 30, 2014 Interest Rate Swap 7 [Member]	Jun. 30, 2014 Interest Rate Swap 8 [Member]	Jun. 30, 2014 Credit Risk Contract [Member]	Jun. 30, 2014 Unsecured Debt [Member] $300 Million 2011 Unsecured Term Loan Maturing 2016 [Member] contract	Jun. 30, 2014 Unsecured Debt [Member] Unsecured 300 million 2013 Term Loan Maturing 2019 [Member] contract	Jun. 30, 2014 Senior Notes [Member] 2014 Senior Notes [Member] contract	Jun. 30, 2014 Senior Notes [Member] 2014 Senior Notes [Member] Forward Starting Interest Rate Swap [Member]
Derivative [Line Items]
Number of forward starting swaps																		4	4
Total notional value	$ 500,000,000								$ 125,000,000	$ 75,000,000	$ 50,000,000	$ 50,000,000	$ 50,000,000	$ 50,000,000	$ 50,000,000	$ 50,000,000		$ 300,000,000	$ 200,000,000		$ 350,000,000
Face amount of debt instrument																		300,000,000	300,000,000	400,000,000
Number of swaps settled				5																5
Settlement of forward starting interest rate swaps	(14,960,000)	(672,000)																		15,000,000
Term of notes																				10 years
Interest Rate Cash Flow Hedge Derivative at Fair Value, Net [Abstract]
Gross derivative assets	0		24,176,000
Gross derivative liabilities	(5,971,000)		(4,526,000)
Net derivative asset/(liability)	(5,971,000)		19,650,000
Amount of gain/(loss) recognized in OCI on derivative					(3,617,000)	22,200,000	(13,502,000)	21,860,000
Amount of previously recorded loss reclassified from accumulated OCI into interest expense					1,159,000	776,000	2,328,000	1,545,000
Loss to be reclassified from accumulated other comprehensive loss to interest expense over next twelve months	4,500,000
Assets needed for immediate settlement, aggregate fair value																	$ 6,100,000